Function decode_split 

pub fn decode_split(
    shared_secret: &[u8],
    sealed: &KkSealedMessage,
    epsilon: &EntropySnapshot,
) -> Result<Vec<u8>>

Decode a split-channel message by reuniting ciphertext with ε.

The receiver needs:

• The shared secret (what both parties know)
• The KkSealedMessage (from Channel 1, the public wire)
• The EntropySnapshot (from Channel 2, the private channel)

All three factors must be present. Missing any one = no decryption.